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e-martin eNotes educator| Certified Educator

This is a poem about the plight of captivity (and oppression) expressed as an emotional reality. The poem uses the metaphor of a physical cage as its principal vehicle to examine the nature of captivity as specific and negative way of relating to the world. 

Contrasting freedom and captivity, the poem draws a comparison between a free bird that is able to not only fly freely in the sky but also to think freely.

This specific element of its freedom allows the bird to be audacious as it 

[...] dares to claim the sky.

A caged bird is presented in stark relief against this emotional, imaginative and physical freedom. In a refrain repeated twice in the poem the caged bird is described with the phrase,

his wings are clipped and his feet are tied   
so he opens his throat to sing.
Significantly, the poem's structure mimics that of a song, using a chorus/refrain the follows the above repeated phrase and appears twice in the poem. 
The caged bird sings   
with a fearful trill   
of things unknown   
but longed for still   
and his tune is heard   
on the distant hill   
for the caged bird   
sings of freedom.
In formatting the poem as a song, the poet implicitly suggests that she is a caged bird and this poem is her song. Thus, the poem's meaning relates to (1) an awareness of what it means to be free in terms of internal life and emotional liberty and (2) a sense that the constrictions of captivity are also emotional. 
The emotional constrictions of the caged bird's situation are made clear in the line 
his bars of rage
which identifies the bars as being made of an emotion, rage. 
One conclusion to draw as to the meaning of the poem comes from addressing the idea that freedom and captivity are not purely physical states. Captivity is an emotional state and so can be produced by situations of oppression. Thus captivity -- being caged -- becomes a metaphor for a perceived or fully felt state of mind wherein an individual feels unable to determine his own path.
But a caged bird stands on the grave of dreams   
his shadow shouts on a nightmare scream 

The song ultimately becomes a statement of yearning that stems from loss. What has been lost is not easily stated because it is, in large part, a way of relating to the world. Yet in phrases like "grave of dreams" the poem suggests that what has been lost can be described as a power to hope, dream or believe in a positive future. A positive, assertive or affirmative relationship with the world has been taken from the caged bird.

user9978515 | Student

it mean about freedom